1. Field of the Invention
This invention relates to a process for producing a high-gloss coated paper, which comprises subjecting to a hot calender treatment a coated paper in which is used an aqueous coating solution comprising a pigment, a synthetic polymer latex (A) having a glass transition temperature of at least 38.degree. C. and a synthetic polymer latex (B) having a glass transition temperature of 5.degree. to 25.degree. C.
2. Description of the Prior Art
High-gloss coated papers can be produced by the application of an aqueous coating solution comprising a pigment and a thermoplastic binder and subsequently imparting gloss to the coated paper by means of hot calendering such as supercalendering or gloss-calendering. The supercalendering for imparting gloss to the coated paper is carried out at a linear pressure of about 200 kg/cm, which is a relatively high nip pressure, at a steel finishing roll temperature of about 70.degree. C. The supercalendering is disadvantageous in that it compresses and densifies the coated paper. On the other hand, a method of finishing a paper surface by means of a hot calender known as "gloss-calendering" comprises pressing a coated paper against a polished finishing drum under temperature conditions sufficient to cause a temporary plastic state on the paper surface, to finish the paper surface. This method provides a coated paper with a bulky quality as compared with supercalendering, whereby a coated paper for printing having excellent opacity is obtained.
The pressure used in the gloss-calender is generally lower than that in the supercalender and is about 90 kg/cm, and the temperature used is as high as about 150.degree. C.
The aqueous coating solution for producing a coated paper usually contains a pigment such as clay, calcium carbonate, aluminum hydroxide, titanium dioxide, satin white or the like, and a binder for sticking the pigment on the paper. As the binder, casein has heretofore been used. In recent years, however, starch is almost always used because casein is expensive and is difficult to make into a solution with a high concentration, and in general a synthetic polymer latex is used together with starch, whereby the low binding strength, low water resistance of the coated surface, and the like which are the disadvantages of starch, are removed. The synthetic polymer latex for coating a paper comprises as essential constituents styrene, butadiene and an acrylic ester, and makers use different mixing proportions and the glass transition temperature Tg of the polymer is generally -20.degree. to 0.degree. C. When a synthetic polymer latex having a Tg of -20.degree. to 0.degree. C., namely lower than 5.degree. C., is used, the coated paper is relatively low in white-paper gloss and print gloss, and no high-gloss paper can be obtained. Furthermore, the coated paper is not sufficient in surface strength in wet state. It is described in U.S. Pat. No. 3,873,345 that in order to produce a high-gloss paper, a synthetic polymer latex having a high Tg of 38.degree. C. or higher is used.
When a synthetic polymer latex having a Tg of at least 38.degree. C. is used and the coated paper is mirror-finished at so high a temperature in the hot calender that the temperature of the coated surface immediately after the finishing becomes a temperature higher than the Tg of the polymer latex having a Tg of at least 38.degree. C. the coated paper is very high in paper gloss, good in ink setting, but the print-gloss is insufficient, and the surface strength is also low and is not sufficient. When the finishing speed is extremely low and the temperature of the hot calender and the pressure under which the coated surface is pressed against the hot calender are extremely high in the mirror-finishing of the coated paper, a sufficient surface strength of the coated paper can be obtained. However, the printability and workability of the coated paper are significantly impaired. That is to say, troubles such as a deterioration in ink setting, an increase in curl, an increase in stain on the hot calender, and the like occur. Therefore, the coated paper obtained under such conditions cannot be used in practice. On the other hand, when the coated paper is finished at such a low temperature in the hot calender that the temperature of the coated surface immediately after the mirror-finishing becomes a temperature lower than the Tg of the synthetic polymer latex having a Tg of at least 38.degree. C., the surface strength of the coated paper is significantly decreased in both dry and wet conditions, and the print gloss is also greatly reduced. When a synthetic polymer latex having a Tg of less than 5.degree. C. and a synthetic polymer latex having a Tg of at least 38.degree. C. are used at the same time and the coated paper is mirror-finished at so high a temperature in the hot calender that the temperature of the coated surface immediately after the finishing becomes a temperature higher than the Tg of the polymer latex having a Tg of at least 38.degree. C., the surface strength of the coated paper increases and becomes substantially satisfactory in the dry condition, but is not satisfactory in the wet condition. The print gloss increases somewhat, but the white-paper gloss is low and the setting is bad.